One-Pot Synthesis of Penta-twinned Palladium Nanowires and Their

Aug 21, 2017 - Zhao, Elnabawy, Vara, Xu, Hood, Yang, Gilroy, Figueroa-Cosme, Chi, Mavrikakis, and Xia. 2017 29 (21), pp 9227–9237. Abstract: Noble-m...
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One-Pot Synthesis of Penta-twinned Palladium Nanowires and Their Enhanced Electrocatalytic Properties Hongwen Huang,†,‡,⊥ Aleksey Ruditskiy,§,⊥ Sang-Il Choi,† Lei Zhang,† Jingyue Liu,∥ Zhizhen Ye,‡ and Younan Xia*,†,§ †

The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States ‡ State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∥ Department of Physics, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *

ABSTRACT: This article reports the design and successful implementation of a one-pot, polyol method for the synthesis of penta-twinned Pd nanowires with diameters below 8 nm and aspect ratios up to 100. The key to the success of this protocol is the controlled reduction of Na2PdCl4 by diethylene glycol and ascorbic acid through the introduction of NaI and HCl. The I− and H+ ions can slow the reduction kinetics by forming PdI42− and inhibiting the dissociation of ascorbic acid, respectively. When the initial reduction rate is tuned into the proper regime, Pd decahedral seeds with a penta-twinned structure appear during nucleation. In the presence of I− ions as a selective capping agent toward the Pd(100) surface, the decahedral seeds can be directed to grow axially into penta-twinned nanorods and then nanowires. The Pd nanowires are found to evolve into multiply twinned particles if the reaction time is extended beyond 1.5 h, owing to the involvement of oxidative etching. When supported on carbon, the Pd nanowires show greatly enhanced specific electrocatalytic activities, more than five times the value for commercial Pd/C toward formic acid oxidation and three times the value for Pt/C toward oxygen reduction under an alkaline condition. In addition, the carbon-supported Pd nanowires exhibit greatly enhanced electrocatalytic durability toward both reactions. Furthermore, we demonstrate that the Pd nanowires can serve as sacrificial templates for the conformal deposition of Pt atoms to generate Pd@Pt core−sheath nanowires and then Pd−Pt nanotubes with a well-defined surface structure. KEYWORDS: penta-twinned nanowires, bimetallic nanotubes, oxidative etching, reduction kinetics, electrocatalytic properties

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INTRODUCTION Palladium nanocrystals have received considerable attention as a primary catalyst for the electro-oxidation of formic acid (FAO) and therefore the fabrication of direct formic acid fuel cells (DFAFCs).1−3 They are also emerging as an alternative to the Pt-based electrocatalyst toward the oxygen reduction reaction (ORR).3 Tremendous efforts have been devoted to the synthesis of Pd nanocrystals with diversified shapes as a means to tailor and optimize their catalytic and electrocatalytic properties through surface structure engineering.4,5 To this end, Pd nanocrystals with a wide variety of shapes, including tetrahedra,6 cubes,7,8 octahedra,9 decahedra,10 icosahedra,10,11 right bipyramids, 12 and penta-twinned nanorods/nanowires,13,14 have all been reported in recent years. Among them, penta-twinned nanowires are particularly attractive because of their unique structural features, including the lattice strain at a twin boundary and the highly anisotropic onedimensional (1D) morphology. It has been reported that the lattice strain can greatly enhance the electrocatalytic activities © 2017 American Chemical Society

toward both FAO and ORR by modifying their surface electronic structures.15,16 In addition, the use of nanowires can also result in substantial improvement in electrocatalytic durability with respect to their 0D counterparts, owing to the suppression of Ostwald ripping, reduction in sintering and detachment from the support, and improvement in resistance toward metal dissolution.17−19 These merits make pentatwinned nanowires a promising candidate for an active and durable electrocatalyst. Like other metals, there is no intrinsic driving force for Pd atoms to grow into nanowires (or nanorods) with a highly anisotropic morphology due to the isotropy in interaction among the atoms.4,20 As a result, it has been difficult to synthesize Pd penta-twinned nanowires as uniform samples in large quantities. Nevertheless, there exist several reports in Received: August 11, 2017 Accepted: August 21, 2017 Published: August 21, 2017 31203

DOI: 10.1021/acsami.7b12018 ACS Appl. Mater. Interfaces 2017, 9, 31203−31212

Research Article

ACS Applied Materials & Interfaces literature. Using a hydrothermal process at 200 °C, Zheng and co-workers demonstrated the synthesis of Pd penta-twinned nanowires with an average diameter of 9 nm and lengths up to 3 μm.13 Although it was a significant advance, the authors offered essentially no discussion of the underlying mechanism. Subsequently, Huang and co-workers also prepared Pd pentatwinned nanowires using a hydrothermal route.14 The authors attributed the success of their synthesis to good control over oxidative etching, caused by O 2 /halides, through the introduction of small organic molecules to scavenge the halide ions, thus limiting their availability in the solution. Similar to what had been established for the synthesis of Ag nanowires,21 the suppression of oxidative etching could protect the twinned seeds from being oxidized and dissolved during the initial stage of the synthesis. However, no mechanistic insight was presented with regard to the formation of decahedral seeds.4,20 Indeed, the formation of decahedral seeds represents a critical step in the synthesis of penta-twinned nanowires. As established in recent studies, the reaction kinetics play a pivotal role in controlling the formation of Pd seeds with a decahedral structure.22 It was further demonstrated that the Pd decahedral seeds could be employed to produce penta-twinned nanorods made of Pd, Ag, and Cu through seed-mediated growth.23−25 In addition to the involvement of a two-step process, the aspect ratios of these nanorods were typically limited to values below 10. In this work, we designed and successfully implemented a one-pot protocol for the facile synthesis of Pd penta-twinned nanowires with an average diameter as thin as 7.8 nm and aspect ratios up to 100. The protocol was based upon the reduction of a Pd(II) precursor by a combination of diethylene glycol and ascorbic acid. By controlling the amounts of NaI and HCl, introduced to form PdI42− species and inhibit the dissociation of ascorbic acid, respectively, the initial reduction rate could be tuned into the right regime for the generation of Pd decahedral seeds. The relatively slow reduction rate associated with PdI42− was also instrumental to the induction and continuation of one-dimensional growth along the pentatwinned axis of a decahedral seed. Furthermore, the I− ions could serve as a selective capping agent toward the Pd(100) surface, thereby promoting the formation of penta-twinned nanorods and then nanowires. When the reaction time was prolonged, however, the nanowires were observed to evolve into multiply twinned particles without defined geometry. Our analysis implies that oxidative etching was responsible for the destabilization of the penta-twinned nanowires when they were suspended in a solution phase containing halide ions and exposed to oxygen from the air at an elevated temperature. We also demonstrated enhanced electrocatalytic activities and durability for the penta-twinned nanowires toward FAO and ORR under an alkaline condition. Besides, using the asprepared Pd nanowires as sacrificial templates, we demonstrated the fabrication of Pd@Pt core−sheath nanowires and Pd−Pt nanotubes.

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Aldrich. Ethylene glycol (EG; lot no. K43B24) was purchased from J. T. Baker. All the chemicals were used as received. Deionized (DI) water with a resistivity of 18.2 MΩ·cm was used throughout the experiments. Synthesis of Pd Nanowires in DEG. In a standard synthesis, 105 mg of PVP, 100 mg of AA, 100 mg of NaI, and 5 μL of aqueous HCl (12 M) were dissolved in 8 mL of DEG hosted in a 24 mL vial and heated at 160 °C in an oil bath for 15 min under magnetic stirring. Subsequently, 3 mL of DEG containing 30 mg of Na2PdCl4 was added in one shot through a pipet. After the reaction had proceeded for 1 h, it was terminated by immersing the vial in an ice−water bath. Finally, the product was collected by centrifugation (12,000 rpm, 10 min), washed with ethanol three times, and re-dispersed in water. Synthesis of Pd@Pt Core−Sheath Nanowires. In a typical procedure, 2.0 mL of the Pd nanowires (0.20 mg/mL), 100 mg of AA, 66.6 mg of PVP, 54 mg of KBr, and 13 mL of EG were mixed in a 50 mL three-neck flask and heated at 110 °C for 15 min under magnetic stirring. The reaction temperature was then quickly ramped to 200 °C within 30 min. Meanwhile, 1.2 mg of Na2PtCl6·6H2O was dissolved in 12 mL of EG and this solution was then injected into the flask using a syringe pump at a rate of 4 mL/h. After completion of the injection, the reaction mixture was kept at 200 °C for another 10 min. The reaction was quenched by immersing the flask in an ice bath. The products were collected by centrifugation (12,000 rpm, 10 min), washed with ethanol three times, and re-dispersed in DI water. Synthesis of Pt-Based Nanotubes. In a typical process, 0.5 mL of the aqueous suspension of Pd@Pt core−sheath nanowires (0.48 mg/mL), 25 mg of FeCl3, 0.15 mL of HCl (12 M), 50 mg of PVP, 300 mg of KBr, and 7 mL of DI water was mixed in a 20 mL vial. The vial was then immersed in an oil bath held at 95 °C under magnetic stirring. After 2 h, the products were collected by centrifugation (12,000 rpm, 10 min), washed with ethanol, and re-dispersed in water. Kinetic Studies of the Pd Nanowire Synthesis. During a standard Pd nanowire synthesis, 100 μL aliquots were drawn with a micropipette in 5 min intervals over 1 h, with the first aliquot drawn immediately after the injection of the Na2PdCl4 precursor. The aliquots were thoroughly mixed with 1.4 mL of ice-cold aqueous NaI solution (200 mg/mL). The resultant mixture was centrifuged at 16,500 rpm for 15 min to remove the solid product. A 100 μL aliquot of the resultant supernatant was injected into 0.9 mL of aqueous NaI (200 mg/mL) inside a cuvette, and the Pd(II) concentration was measured using UV−vis spectroscopy. Electrochemical Measurements for FAO and ORR under an Alkaline Condition. Prior to the electrochemical measurements, the carbon-supported Pd nanowires (denoted as Pd nanowires/C) with a Pd content of 20 wt % (determined by ICP-MS) were prepared by loading the as-prepared nanowires on a carbon support (Vulcan XC72). The catalyst ink was then prepared by ultrasonically mixing of the catalyst with 960 μL of isopropyl alcohol and 40 μL of 5 wt % Nafion solution for 1 h. The working electrode was obtained by drop-casting 10 μL of an aqueous suspension of the catalyst ink onto a precleaned, glassy carbon electrode (geometrical area of 0.196 cm2, Pine Instruments). The loading amount of metal Pd or Pt for each catalyst was kept at 3 μg. The electrochemical measurements were carried out with a three-electrode system on an IM6 electrochemical workstation (Zahner, Germany). A Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. All potentials were converted to values with reference to reversible hydrogen electrode (RHE). To remove the surface ligand, such as I− ions, the Pd nanowires/C loaded on electrodes were cycled between 0.08 and 1.5 V versus RHE (denoted VRHE) for 400 cycles in a N2-saturated aqueous solution containing HClO4 (0.1 M) at a scan rate of 50 mV s−1. The activities of Pd nanowires/C and commercial Pd/C (10 wt % Pd loading, Sigma) for FAO were obtained by cycling the potential between 0.08 and 1.1 V for three cycles in a N2-saturated aqueous solution containing HClO4 (0.5 M) and HCOOH (0.5 M) at a scan rate of 50 mV s−1. For ORR under an alkaline condition, the cyclic voltammograms (CVs) for Pd nanowires/C and commercial Pt/C catalyst (20 wt % Pt loading, Johnson Matthey) were measured in a N2-saturated KOH (0.1 M) solution between 0.2 and 1.2 VRHE at a sweep rate of 50

EXPERIMENTAL DETAILS

Chemicals and Materials. Diethylene glycol (DEG; ≥99.0%, lot no. BCBK6125 V), sodium tetrachloropalladate(II) (Na2PdCl4; 99.998%), hexachloroplatinate(IV) hexahydrate (Na2PtCl6·6H2O; 98.0%), poly(vinylpyrrolidone) (PVP; MW ≈ 55,000), aqueous HCl solution (37 wt % or 12 M), iron(III) chloride (FeCl3; 97%), Lascorbic acid (AA; ≥99.0%), sodium iodide (NaI; ≥99.5%), and potassium bromide (KBr; 99.0%) were all obtained from Sigma31204

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ACS Applied Materials & Interfaces mV s−1. The ORR measurements for Pd nanowires/C and commercial Pt/C were performed using the rotating disk electrode (RDE) method in O2-saturated 0.1 M KOH at a rotation rate of 1,600 rpm and a sweep rate of 10 mV s−1. The accelerated durability tests (ADTs) were performed at room temperature by applying cyclic sweeps between 0.4 and 1.0 VRHE at a sweep rate of 100 mV s−1 for 10,000 cycles. The electrochemically active surface areas (ECSAs) of the carbonsupported Pd nanowires and commercial Pd/C catalyst were calculated from the CVs for Cu underpotential deposition (CuUPD), which were conducted in aqueous solutions containing H2SO4 (0.05 M) and CuSO4 (0.05 M) at a scan rate of 5 mV s−1. Consistent with previous studies, the ECSA of commercial Pt/C in alkaline media was estimated by integrating the hydrogen desorption regime in CV of commercial Pt/C.26 Instrumentation. The X-ray diffraction (XRD) patterns were obtained on a diffractometer (Paralytical XRD-600) operated at 40 kV and 40 mA with filtered Cu Kα radiation at λ = 0.154 nm. The transmission electron microscopy (TEM) images were obtained using a Hitachi HT7700 microscope operated at 120 kV. The highresolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images were acquired using a JEOL JEM 2200FS STEM/TEM microscope equipped with a CEOS (Heidelberg, Germany) probe corrector to provide a nominal image resolution of 0.07 nm. Elemental analysis of products was conducted using an inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer, NexION 300Q). UV−vis absorption spectra were recorded with a Lambda 750 spectrometer (PerkinElmer).

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RESULTS AND DISCUSSION Characterizations of the Pd Nanowires. We first analyzed the as-obtained nanowires using XRD. As shown in Supporting Information Figure S1, all the peaks could be indexed to the diffraction from fcc Pd (JCPDS No. 46-1043). No other crystalline phases such as PdO were detected. As shown in the TEM image (Figure 1A), the Pd nanowires had an average length of 720 nm, together with a purity over 85%. Figure 1B shows another TEM image of the Pd nanowires recorded at a higher magnification, highlighting good uniformity along each nanowire, together with an average diameter of 7.8 nm. The HAADF-STEM image in Figure 1C further confirmed the good uniformity in diameter along the long axis of an individual nanowire. A closer look at the nanowire ends (Figure 1D) revealed the presence of a pentatwinned structure. A straight twin boundary could be seen running along the length of the nanowire, as marked by the red arrow. The lattice fringes only appeared on one side of the imaged nanowire, indicating that the direction of the incident electron beam was parallel to one of the five side faces, as schematically illustrated in the inset of Figure 1D. Such an orientation corresponds to the superposition of ⟨110⟩ and ⟨111⟩ zones.27 The HRTEM image taken from the middle section of a nanowire (Figure 1E) clearly shows lattice fringes with spacing of 0.23 and 0.20 nm, which could be indexed to the {111} and {200} planes of fcc Pd, respectively. The Fourier transform (FT) pattern displayed in Figure 1F could be assigned to two sets of diffraction patterns corresponding to the ⟨110⟩ and ⟨111⟩ zones of Pd, further confirming the existence of twinned planes in the nanowire. Taken together, we can conclude that the Pd nanowires possessed a penta-twinned structure bound by 10 {111} facets at the two ends and five {100} side facets, which is a common feature of onedimensional metal nanostructures composed of fcc metals such as Cu, Ag, and Au.27,28 Effect of Reduction Kinetics on the Formation of Pd Nanowires. In order to elucidate the driving forces behind the

Figure 1. Morphological and structural characterization of the Pd nanowires obtained using the standard procedure. (A, B) TEM images at low and high magnifications, respectively, and (C) HAADF-STEM image. (D, E) HRTEM images of the region marked by a dashed box in panel C and the middle section of a nanowire, respectively. The red arrow and dotted line indicate the twin boundary running along the long axis of the nanowire. The inset in panel D illustrates the orientation of the electron beam relative to the nanowire. (F) Fourier transform pattern derived from the lattice fringes in panel E.

initial formation of Pd decahedral seeds, we studied the reaction kinetics underlying the nanowire synthesis. Previous work has shown that the type of seed generated during the nucleation phase can be controlled by tuning the initial reduction rate of a salt precursor into the proper regime.22,29 The synthesis of Pd nanowires is a significantly more complicated system compared to those described in the previous reports on this subject. Specifically, in the nanowire synthesis, both DEG and AA served as reducing agents, with the reducing power of AA dependent on the concentration of HCl. However, since both of these components are in excess when compared to the Pd(II) precursor, we can approximate the reaction system as pseudo-first order. To quantify the kinetic parameters for the nanowire synthesis, we used UV−vis analysis to measure the concentration of Pd(II) precursor remaining in the solution over the course of reaction. The experiment was conducted for both the standard synthesis and a modified procedure in which the I− was substituted by a molar equivalent of Br− in order to quantify the impact of a specific halide on the reaction kinetics. Figure S2A shows a TEM image of the Pd nanostructures 31205

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ACS Applied Materials & Interfaces obtained by replacing I− with Br− in the standard synthesis. The products consisted of a mixture of decahedra (marked by the blue arrows), truncated right bipyramids (marked by the red arrows), and nanocubes. To better distinguish the shapes of a decahedron and a truncated right bipyramid, their corresponding 3D models are presented in Figure S2B,C, respectively. When the initial PdCl42− precursor is exposed to an excess amount of Br− or I−, rapid ligand exchange will occur, generating a PdI42− or a PdBr42− complex. This change is driven by the greater stability of the resultant complexes, with the stability trending in the order of PdCl42− < PdBr42− < PdI42−.30 In the case of standard synthesis, the characteristic absorption peak of PdI42− at 408 nm was used to track the Pd(II) precursor concentration, while for the modified procedure the 332 nm absorption peak corresponding to PdBr42− was utilized. The reaction was sampled in 5 min intervals for the duration of 1 h. The concentration of the Pd(II) precursor remaining in solution is directly proportional to the absorbance of the characteristic peak. Using this information, the rate constant for the reduction can be obtained by plotting the integrated form of the pseudo-first-order reaction rate law: ln[A]t = −kt + ln[A]0

faster reduction in the bromide-mediated system. Notably, the rate constant of the iodide-mediated reaction was nearly identical to that of the sulfate-mediated synthesis of Pd decahedra (5.95 × 10−5 s−1), which was previously reported by our group.22 This result strongly suggests that Pd decahedral seeds are created in the initial stage of a nanowire synthesis, and the seeds subsequently evolve into pentagonal nanowires through axial overgrowth. When the k values are multiplied by the initial concentration of PdCl42− (9.27 × 10−3 M), we obtain the initial reduction rates (ro): 4.47 × 10−7 and 1.62 × 10−5 M s−1 for the iodide- and bromide-mediated syntheses, respectively. To further study the impact of the reduction kinetics, we varied the concentrations of Na2PdCl4 and NaI. As shown in Figure 3A, increasing the amount of Na2PdCl4 to 45 mg

(1)

where [A]0 and [A]t represent the concentrations of the Pd(II) precursor at the beginning of a synthesis and at a specific time point, respectively; k is the rate constant; and t is time. The derived plots for both the iodide- and bromide-mediated nanowire syntheses are shown in Figure 2. The iodide-mediated

Figure 3. TEM images of Pd nanostructures obtained using the standard procedure with the addition of different amounts of Na2PdCl4, (A) 45 and (B) 25 mg, respectively; and different amounts of NaI, (C) 150 and (D) 50 mg, respectively.

Figure 2. Plot showing the natural log of Pd(II) concentration over time for the iodide- and bromide-mediated nanowire syntheses. For the bromide-mediated synthesis, the characteristic peak of PdBr42− disappeared after 15 min due to the involvement of rapid reduction. The rate constant (k) and initial reduction rate (ro) are labeled on the fitting curves.

produced a mixture of Pd nanocubes (with an average size of 14 nm) and right bipyramids (with an average size of 30 nm). When the amount of Na2PdCl4 was decreased to 25 mg, uniform Pd nanowires were obtained (Figure 3B). Further decreasing the amount of Na2PdCl4 to 15 mg resulted in no solid product in the reaction system, due to the very slow reduction rate. Variation in the amount of NaI added could also be used to adjust the reaction kinetics. Since PdI42− is more stable than PdCl42−, a high concentration of NaI would appreciably slow the reduction rate.31,32 As illustrated in Figure 3C,D, short Pd nanowires, as well as right bipyramids and small amounts of nanocubes, were produced if 150 mg of NaI was used. In contrast, the product contained 15% of right bipyramids and 85% of nanocubes when the amount of NaI was reduced to 50 mg. All these results consistently indicate that the formation of Pd nanowires requires a relatively slow reduction rate for the precursor. It should be noted, however, that changing the concentration of I− not only impacts the

synthesis showed a slow, linear decline in ln[Pd2+] over the course of 1 h, as expected from a pseudo-first-order reaction. On the other hand, the decline in ln[Pd2+] for the bromidemediated reaction was comparatively rapid, with the characteristic absorption peak disappearing after 15 min of reaction. This result suggests a significantly more rapid reduction rate in the case of the bromide-mediated system, which is consistent with the relative stabilities of the PdI42− and PdBr42− complexes. The k values, derived from the slope of the fitted lines, were 4.82 × 10−5 and 1.75 × 10−3 s−1 for the iodide- and bromide-mediated reactions, respectively. The more than 2 orders of magnitude difference between the rate constants confirms the significantly 31206

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ACS Applied Materials & Interfaces reduction kinetics but also influences the capping of Pd(100) facets and the rate of oxidative etching. On the basis of the experimental investigations described above, we can posit a mechanism for the formation of the Pd nanowires, summarized in Figure 4. Immediately after the

Figure 4. Schematic illustration of the nucleation and growth pathway for the formation of Pd nanowires with a penta-twinned structure. Homogeneous nucleation of Pd atoms initially produces decahedral seeds. The capping of the Pd(100) facets by I− induces selective deposition of Pd atoms on the Pd(111) facets, resulting in the formation of pentagonal nanowires.

injection of the Na2PdCl4 precursor into the reaction solution, the PdCl42− complex is converted to PdI42− because of ligand exchange with the excess I−. The PdI42− complex then undergoes reduction by the combination of DEG and AA, producing Pd(0) atoms. The reducing power of AA is moderated through the introduction of H+ ions from HCl, which serve to inhibit the dissociation of AA into ascorbate monoanion, which is a much stronger reducing agent than AA. When the concentration of Pd(0) atoms reaches supersaturation, homogeneous nucleation occurs, resulting in the formation of decahedral seeds. The remaining I− ions act as a capping agent to passivate the Pd(100) facets of the decahedra, as previously observed for the seeded growth of Pd nanorods.23 Consequently, the subsequent nucleation of Pd(0) is heterogeneous, occurring on the Pd(111) surfaces of the decahedra. Additionally, the slow reduction of PdI42− limits the supply of Pd(0), further promoting the asymmetric overgrowth of the decahedra. The result is the axial growth of the decahedra along the ⟨110⟩ direction, thereby generating pentagonal nanorods and, after further growth, nanowires. This mechanism is consistent with those proposed for the syntheses of Cu and Ag nanowires with a pentagonal cross-section.21,27,28 Oxidative Etching and Stability of the Pd Nanowires. As mentioned in the Introduction, previous attempts to synthesize Pd nanowires showed product instability at long reaction times. In order to follow the structural evolution of Pd nanowires, we characterized the products sampled at different stages of a synthesis by TEM. At t = 0.5 h (Figure 5A), the sample mainly contained nanowires with an average diameter of 7.5 nm and an average length of 450 nm. As the reaction time was extended to t = 1.5 h, however, a small number of irregular particles appeared, as shown in Figure 5B. These particles exhibited a tadpole-like geometry (with one end being significantly larger than the other), together with a multiply twinned structure. Upon closer examination by TEM (Figure S3), it was found that most of these particles were positioned at the ends of short Pd nanowires. As the reaction continued, the number and size of such irregular particles also increased. At t = 2 h (Figure 5C), the tadpole-like particles grew to 200−400 nm in length while the average length of the nanowires could reach 2 μm. At t = 4 h (Figure 5D), essentially all of the Pd nanowires disappeared, with only multiply twinned particles remaining in

Figure 5. TEM images of the samples obtained at various reaction times for a standard nanowire synthesis: (A) 0.5, (B) 1.5, (C) 2, and (D) 4 h, respectively.

the sample. The XRD pattern (Figure S4) recorded from these multiply twinned particles only showed peaks corresponding to fcc Pd, indicating that the particles and nanowires had the same composition. These observations clearly indicate that the pentatwinned Pd nanowires were not stable in the reaction solution under ambient conditions at an elevated temperature. Zheng and co-workers also reported that their Pd nanowires would first shrink to Pd nanorods and then transform into multiply twinned particles as the reaction time was extended to 24 h.13 However, no detailed mechanistic discussion was provided in that report. The intrinsic instability of the penta-twinned Pd nanowires is likely responsible for the limited number of reports on their synthesis in the literature. To elucidate the mechanism responsible for the instability of Pd nanowires, we designed and conducted a set of experiments. To simplify the study, we dispersed the purified Pd nanowires prepared using the standard procedure in DEG containing different chemical species and then monitored their morphological changes as a function of aging time. As shown in Figure 6A, the shape of the nanowires was retained after aging at 160 °C for 4 h in the presence of PVP (15 mg/mL). When the aging time was extended to 12 h, short nanorods with varying diameters and rough surfaces were observed (Figure 6B). In contrast, most of the Pd nanowires were quickly broken into small particles after aging in a DEG solution containing NaI (50 mg/mL), PVP (15 mg/mL), and HCl (30 mM) for just 10 min (Figure 6C). If the aging time was increased to 60 min, Pd particles much larger in size would be formed, as shown in Figure 6D. The TEM image in the inset reveals that the asobtained particles possessed a multiply twinned structure. These observations indicate that the presence of NaI and HCl was responsible for the instability of the penta-twinned Pd nanowires suspended in a solution phase. Similar to Cl−, I− contributes to the oxidative etching of Pd nanostructures by promoting the formation of a PdI42− stable complex and acting 31207

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nanowires, causing fragmentation of the Pd nanowires into short nanorods and small particles, as the synthesis was continued. On the basis of experimental observations described above, we can propose a plausible mechanism for the time-dependent shape evolution of Pd nanowires. Pd nanorods are formed from initial decahedral nuclei, followed by their growth along the ⟨110⟩ direction into longer nanowires. At longer time periods, two reactions occur in solution. The first is the reduction of Pd2+ ions into Pd(0) atoms and their subsequent deposition onto the {111} facets at the ends of the Pd nanowires, which results in the longer nanowires observed in Figure 5C. The second is the fracturing of the Pd nanowires into smaller parts, such as nanorods, through oxidative etching. We expect that the newly formed fracture is enclosed by high-index facets, rather than the {111} facets found at the ends of standard nanowires. The preferential deposition of Pd at these high-energy sites serves to lower the total surface energy, resulting in the formation of a roughly spherical segment at the site of fracture. Upon further surface diffusion and Pd atom deposition the tadpole-like particle is subsequently created. Eventually, all of the nanowires are transformed into the irregular, twinned particles owing to the combination of oxidative etching, reduction of the released Pd2+ ions, and their deposition on the surface of the irregular particle, as shown in Figure 5D. It is well-known that the strength of oxidative etching is strongly correlated with the reaction temperature.35 Therefore, as the reaction temperature is decreased, the rate of oxidative etching should be observably slowed. On the basis of this argument, we carried out another series of experiments conducted at 150 °C to verify the proposed mechanism for the shape evolution of the Pd nanowires. As expected, Pd nanowires, with average length of 350 nm and average diameter of 8 nm, were obtained at t = 1 h, as shown in Figure S5A. After extending the reaction time to t = 6 h, Pd nanowires and tadpole-like particles were produced (Figure S5B). Because of the relatively slow rate of the oxidative etching at lower temperature, when compared to standard procedure, the Pd nanowires could be retained even after t = 6 h. Evaluation of Electrocatalytic Properties. The electrocatalytic properties of the penta-twinned Pd nanowires toward FAO and ORR in alkaline media were then evaluated. Prior to the electrochemical measurements, we prepared Pd nanowires/ C catalysts by loading the as-prepared samples on Vulcan XC72 carbon. For FAO, we benchmarked the electrocatalytic properties of the Pd nanowires/C against a commercial Pd/C under identical conditions. In the first step, the CVs of the Pd nanowires/C and commercial Pd/C for the deposition and stripping of a Cu monolayer underpotentially deposited (UPD) were collected, as shown in Figure S6. The specific ECSAs of the Pd nanowires/C (26.2 m2 gPd−1) and commercial Pd/C (44.9 m2 gPd−1) were then obtained by normalizing the ECSAs, which were calculated by integrating the corresponding stripping charges for CuUPD (420 and 460 μC cm−2 for the Pd nanowires and Pd/C, respectively), against the Pd mass.36 The electrocatalytic activities of the catalysts were further evaluated by recording CV curves in N2-saturated solutions containing 0.5 M HClO4 and 0.5 M HCOOH at a scan rate of 50 mV s−1. Figure 7A shows the CVs for FAO after normalizing to the corresponding ECSAs of the Pd nanowires/C and Pd/C. The Pd nanowires/C showed a specific current density of 13.89 mA cm−2, more than five times the value for commercial Pd/C (Table S1). Such a great enhancement can be attributed to the

Figure 6. TEM images of the products obtained by aging the Pd nanowires at 160 °C. The nanowires were dispersed in different solutions and aged under various conditions: (A, B) in DEG containing PVP (15 mg/mL) for 4 and 12 h, respectively; (C, D) in DEG containing NaI (50 mg/mL), PVP (15 mg/mL), and HCl (30 mM) for 10 and 60 min, respectively; (E) in DEG containing NaI (50 mg/mL), PVP (15 mg/mL), and HCl (30 mM) for 60 min, with argon being bubbled into the solution. (F) TEM image of the product shown in panel E at a higher magnification. The dashed boxes in panel F highlight the roughened side surface and fragmentation of the Pd nanowires caused by the aging.

as a charge carrier, according to the following reaction equation:10,33,34 Pd + O2 + 4I− + 4H+ → PdI4 2 − + 2H 2O

(2)

From eq 2, it is clear that O2 could act as an oxidant and play a critical role in destabilizing the Pd nanowires. To validate the proposed mechanism, we bubbled argon through the DEG solution containing PVP (15 mg/mL), NaI (50 mg/mL), and HCl (30 mM). Figure 6E shows a TEM image of the products obtained after aging at 160 °C for 60 min, revealing that most of the Pd nanowires were retained. Although argon was bubbled into the DEG system, there was still a minor oxidative etching effect, as shown in Figure 6F. Some of the nanowires were found to have roughened side faces (boxed by the large dashed rectangle) as well as fractures (boxed by the small dashed rectangle). Considering these observations, we believe that the oxidative etching likely began at the high-energy twinned boundaries running along the sides of the Pd 31208

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Figure 7. Electrocatalytic performance of Pd nanowires/C toward FAO and ORR. (A) CVs of Pd nanowires/C and commercial Pd/C for FAO recorded in N2-saturated aqueous solutions containing 0.5 M HClO4 and 0.5 M HCOOH at a scan rate of 50 mV s−1. The current density was normalized to the corresponding ECSA. (B) Positive-going ORR polarization curves of the Pd nanowires/C and commercial Pt/C recorded in O2saturated 0.1 M KOH aqueous solutions with a sweep rate of 10 mV s−1. (C) Mass and specific activities of the carbon-supported Pd nanowires and commercial Pt/C at 0.9 VRHE. (D) Comparison of mass activities for Pd nanowires/C and commercial Pt/C at 0.9 VRHE before (left column) and after (right column) 10,000 cycles of ADTs. The color scheme in panel B also applies to panels C and D.

advantageous structural features of Pd nanowires, including the surface strain and exposed {100} facets on the side faces.15 We also conducted a durability test for the Pd nanowires/C and Pd/C in solutions containing 0.5 M HCOOH and 0.5 M HClO4 for 1,000 s with the potential fixed at 0.4 VRHE. The chronoamperometric curves show that the initial and final current densities of the Pd nanowires/C were both much higher than those of Pd/C, indicating a great improvement in durability (Figure S7). For ORR under an alkaline condition, the electrocatalytic properties of Pd nanowires/C were compared with commercial Pt/C under the same measurement conditions. The CVs of Pd nanowires/C and commercial Pt/C shown in Figure S8 were recorded at room temperature in N2-saturated 0.1 M KOH solutions at a sweep rate of 50 mV s−1. Because hydrogen can penetrate into the Pd lattice, no obvious peak was observed in the hydrogen adsorption/desorption region for the CVs.37 From the CV of commercial Pt/C catalyst, the specific ECSA (39.7 m2 gPt−1) was determined by normalizing the ECSA, which can be derived from the charges associated with the desorption of hydrogen, against Pt mass. The electrocatalytic activities of Pd nanowires/C and commercial Pt/C were further measured using the RDE method. Figure 7B shows the positive-going ORR polarization curves of Pd nanowires/C and commercial Pt/C obtained at room temperature in O2-saturated 0.1 M KOH solutions at a rotation speed of 1,600 rpm. The positive potential shift for Pd nanowires/C implies the enhancement in catalytic activity when compared to commercial Pt/C. The mass and specific activities of Pd nanowires/C and commercial Pt/C at 0.9 VRHE

(Figure 7C) were further derived by normalizing the kinetic current densities, which can be calculated by following the Koutecky−Levich equation, against the corresponding metal mass and ECSA, respectively. Specifically, the mass and specific activities of Pd nanowires/C were 0.22 A mgPd−1 and 0.83 mA cm−2, which were 2.0 and 3.0 times as high as those of commercial Pt/C catalyst (mass activity of 0.11 A mgPt−1 and specific activity of 0.28 mA cm−2), respectively. The much higher activity of Pd nanowires/C is believed to originate from their unique structural characteristics, including, for example, surface strain, ultrathin diameter, and 1D morphology, which can optimize the surface electronic structure and thus boost the activity toward ORR.15,16,38 The long-term durability is another important criterion when screening a potential catalyst for practical applications. We thus further evaluated the durability of the catalysts through an ADT at room temperature. As shown in Figure S9 and Figure 7D, the Pd nanowires/C only showed a drop of 13.8% in mass activity after 10,000 cycles of ADTs, against a loss of 31.8% for the commercial Pt/C, indicating an excellent catalytic durability toward ORR in alkaline media. The TEM images in Figure S10 compare the morphologies of Pd nanowires/C and commercial Pt/C before and after 10,000 cycles of ADTs. Compared to the serious sintering for commercial Pt/C, the Pd nanowires/C well-retained their highly anisotropic morphology after the ADTs, demonstrating their superior structural stability. We argued that such a proponent structural stability was responsible for their remarkable long-term durability. Syntheses of Pd@Pt Core−Sheath Nanowires and Pd−Pt Bimetallic Nanotubes. Pt-based one-dimensional 31209

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which is the approximate thickness of the Pt shell deposited on the Pd nanowires.

nanostructures have been demonstrated as effective electrocatalysts toward the oxygen reduction reaction (ORR).39−41 However, it remains a challenge to produce Pt-based onedimensional nanostructures. To this end, we employed the Pd nanowires, obtained using the standard procedure, as seeds to generate the Pd−Pt bimetallic one-dimensional nanostructures. The seeded overgrowth was conducted by injecting Na2PtCl6 with a syringe pump into an EG-based mixture of PVP, AA, KBr, and Pd nanowires at 200 °C (see Experimental Section for details). As shown in Figure 8A, bimetallic nanowires with an

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CONCLUSION In summary, we have successfully synthesized ultrathin, pentatwinned Pd nanowires, with an average diameter of 7.8 nm and an average length of up to 720 nm using a one-pot approach. Adjusting the reduction rate of the Pd(II) precursor into the appropriate regime, in combination with Pd(100) capping by I− ions, was found to be crucial to the success of this synthesis. The Pd nanowires were found to evolve into irregular, multiply twinned particles if the reaction time was prolonged beyond 1.5 h. Further investigation revealed that this instability originated from oxidative etching induced by the O2 from the air in combination with I− ions. The as-prepared Pd nanowires were also demonstrated to possess promising electrocatalytic properties toward the FAO and ORR in alkaline media. Finally, Pd@ Pt core−sheath nanowires were synthesized by using the Pd nanowires as templates. Subsequent selective removal of Pd from these bimetallic structures produced well-defined Pd−Pt bimetallic nanotubes. We believe that the extensive understanding of the formation of Pd penta-twinned nanowires, as well as the facile approach to the production of Pd and Pd−Pt one-dimensional nanostructures, presented here can greatly advance the utility of noble-metal nanocrystals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12018. Catalytic activities comparison; weight percentages; XRD patterns; TEM images; low- and high-magnification TEM images; CVs; chronoamperometric curves; and ORR polarization curves (PDF)

Figure 8. Morphological and compositional characterization of the Pd@Pt core−sheath nanowires, as well as the Pd−Pt nanotubes prepared by selectively etching away the Pd cores. (A) TEM image and (B) EDX mapping of the Pd@Pt core−sheath nanowires. (C) Low-magnification and (D) high-magnification TEM images of the Ptbased nanotubes. The inset in panel D shows a HRTEM image of the boxed region.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

average diameter of 10.3 nm were obtained. The thicker diameter of the bimetallic nanowires corresponds to the deposition of Pt shells onto the surface of the Pd nanowires. EDX mapping shown in Figure 8B confirms the formation of Pd@Pt core−sheath nanowires. Since Pt is more resistant to chemical oxidation when compared to Pd, we could selectively remove the Pd from the Pd@Pt core−sheath nanowires by using the Fe3+/Br− etchant pair (see Experimental Section for details). As shown in Figure 8C,D, hollow nanotubes were obtained after selectively etching the Pd cores from the Pd@Pt core−sheath nanowires. The composition of the nanotubes was subsequently determined by ICP-MS. As shown in Table S2, the significantly lower weight percentage of Pd in the final nanotubes, when compared to the initial nanowires, demonstrates the selective removal of Pd from the Pd@Pt core− sheath nanowires. Notably, a significant amount of Pd was retained in the nanotubes after etching, suggesting the formation of a Pd−Pt alloyed shell. The inset in Figure 8D shows a HRTEM image captured at a section of a nanotube wall (marked by the dashed box in Figure 8D). The wall was composed of six atomic layers with a thickness of about 1.2 nm,

Aleksey Ruditskiy: 0000-0002-1146-827X Younan Xia: 0000-0003-2431-7048 Author Contributions ⊥

H.H. and A.R. made equal contributions to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported in part by a research grant from the NSF (DMR 1506018) and startup funds from the Georgia Institute of Technology. As a visiting Ph.D. student from Zhejiang University, H.H. was also partially supported by a Fellowship from the China Scholarship Council. A.R. was supported by a Graduate Research Fellowship from the NSF. J.L. gratefully acknowledges the support by Arizona State University and the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University. 31210

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